In order to produce wood-based materials with a long service life, it has been known to chemically modify the wood and in particular to acetylate the wood. Thereby materials with improved material properties, e.g. dimensional stability, hardness, durability, etc., are obtained.
In the art, it is known to use batch processes, i.e. stop/start processes for the acetylation of wood particles. Such processes, however, often lead to poor product uniformity as the properties differ significantly from batch to batch and no two batches can ever be considered as identical.
Some references address the option to conduct wood acetylation in a continuous process. Thus, e.g., EP 757570 discloses a process for the acetylation of lignocellulosic materials (LM) comprising a first step wherein the LM is brought into intimate contact with an acetylating agent comprising acetic anhydride as the major component at a temperature from 80° C. to 140° C. and bringing the acetylated LM from the first step into contact with a heated gas inert under the reaction conditions in a stripper at a temperature above 140° C. A comparable process is described in EP 650 998.
To the extent that these references would enable conducting a continuous process, this is on the basis of a plug screw feeder type of impregnation in series with continuous further processing (e.g. reaction). This is described for wood elements of relatively small dimensions, e.g., fibres, small particles, etc. When used for wood elements of larger dimensions, such as chips or strands, the plug screw will normally lead to a product wherein the wood dimensions have become smaller (crushed chips, crushed strands, broken fibres, smaller particles etc.). Particularly, if chips or strands are used, no plugging effect can be achieved to separate between zones with different pressures and/or atmospheres without destruction/crushing of the chips or strands. It is desired to provide a process for the continuous acetylation of wood particles, and particularly of wood chips or strands, wherein the particles are not damaged in the way as done when using a plug screw feeder.
Moreover, in the acetylation of wood elements, particularly of wood chips, a technical challenge in providing a suitable continuous process, is to combine the advantages of a continuous process, with the desired result of a sufficiently high degree and uniformity of acetylation.
The foregoing is addressed in WO 2013/139937. Therein a process is disclosed that judiciously combines a technique for batch-wise impregnation in such a way that the subsequent acetylation can be conducted in a continuous manner. Accordingly, a plurality of batches of wood elements is impregnated with an acetylation fluid, a plurality of batches of impregnated wood elements is collected in such a manner as to provide a continuous feed of impregnated wood elements to a reactor, as a result of which a continuous flow of impregnated wood elements to acetylation reaction conditions, the output of the process being a continuous flow of acetylated wood elements.
It is notoriously difficult to obtain high acetylation degrees for wood elements. In WO 2013/139937 this is successfully addressed. Without wishing to be bound by theory, the present inventors believe that the impregnation step thereby is key. Thereby the impregnation preferably is conducted as a Bethel type of wood impregnation process. In the Bethel process, the wood is placed in a vacuum chamber and vacuum is applied to draw air from the wood. Active ingredients (in the present invention: an acetylation fluid) is then added to the chamber under vacuum. After filling the chamber with liquid a pressure generally up to 250 pounds per square inch (psi) can be applied, preferably 150 psi to 200 psi. The pressure is removed so that the wood is again subject to atmospheric pressure. This type of process is preferred, since it should normally result in a maximum impregnation load, which is believed to have a direct relation to a desired maximum acetyl level.
In the art, Bethel type impregnation essentially is a batch process. Whilst WO2013/139937 presents an elegant and suitable way of achieving the optimal impregnation batchwise, and combine this with continuous acetylation, it would be desired to provide the possibility of a continuous impregnation of the Bethel type.
The present invention aims to provide a continuous process for the impregnation of wood elements with acetylation fluid, thereby securing the concomitant advantages of consistency in quality (such as acetyl levels, and preferably also residual acetic acid and residual acetic anhydride level) and cost over existing batch processes. Particularly, the invention also aims to provide a continuous impregnation and acetylation process that enables the production of all wood elements having a relatively high degree of acetylation.